In the field of POD (Print On Demand) or shortrun printing, continuous print preparation of several thousand print sheets is to be estimated, since print preparation scale thereof is much larger than that of office use. In such the printing case, stability obtained via no variation of image quality from beginning to end has been demanded. Further, in this field, there was also an opportunity to prepare print sheets in which a great variety of information was included, and toner consumption was often changed largely with variation in picture element ratio of the image during print preparation.
In the case of full-color image formation, image formation by a two-component developing system employing a developer composed of a toner and a carrier is widely utilized, but the two-component developer capable of enduring ling-term use has been demanded in order to stably provide full-color images with excellent image quality.
In the two-component developing system, the toner is appropriately charged via friction of the toner with the carrier surface, but when developing is repeatedly conducted over a long period of time, a resin layer provided on the carrier surface is peeled because of the friction, and toner constituents adhere to the carrier surface, whereby charging ability of the carrier is gradually lowered. As the result, there appeared a problem such that developing performance to supply the predetermined amount of toner from the carrier to a photoreceptor was degraded, whereby image density was varied, and specifically, the desired color balance was difficult to be obtained in full-color image formation.
Further, also in the case of continuous print preparation of several thousand print sheets, demanded has been print preparation performance so as to suppress and minimize change in image density and hue of all the print sheets printed from beginning to end.
Consequently, proposed is an image forming apparatus equipped with a so-called trickle developing system developing device by which a replenisher developer, in which a small amount of new carrier is mixed with a new toner, is supplied from a replenishing developer container to a developing device, and charging ability of the carrier is maintained by gradually replacing in-use carrier with the new carrier (refer to Patent Documents 1 and 2, for example). The technique disclosed in Patent Document 1 relates to a color image forming apparatus equipped with a trickle developing system developing device by which a replenisher developer prepared via mixture so as to make the carrier concentration to be about 3% to about 50% is supplied.
The technique disclosed in Patent Document 2 is one by which three kinds of replenisher developers in which each of three kinds of carriers having different resistance is prepared with the toner are stored by dividing an area into three regions in the longitudinal direction of a developer container, and a new carrier having different resistance is sequentially supplied as toner replenishing time passes.
In view of a long-term outlook, the ability decline of carrier can be inhibited by replenishing the degraded carrier with a new carrier. On the other hand, the carrier concentration (a ratio of carrier weight to replenisher developer weight) of a replenisher developer sequentially replenished momentarily from the developer container to the inside of the developing device is not stable, and largely varied.
For this reason, there appeared a problem such that toner supply performance of the developer supplied onto a developing roller facing a latent image on a photoreceptor was largely varied, and difference in performance was produced at the position of the developing roller, whereby image density unevenness at solid density image portions, or image roughness at halftone image portions was generated.    (Patent Document 1) Japanese Patent O.P.I. Publication No. 11-223960    (Patent Document 2) Japanese Patent O.P.I. Publication No. 2004-29306